The present invention relates to novel xcex2-lactams. The xcex2-lactams of the present invention find utility as intermediates in the preparation of sidechain-bearing taxanes such as taxol and taxol derivatives. The present invention also relates to novel methods of coupling xcex2-lactams to form such sidechain-bearing taxanes, and to novel sidechain-bearing taxanes.
Taxanes are diterpene compounds having utility in the pharmaceutical field. For example, taxol, a taxane having the structure: 
where Ph is phenyl, Ac is acetyl and Bz is benzoyl, has been found to be an effective anticancer agent.
Naturally occurring taxanes such as taxol may be found in plant materials, and have been isolated therefrom. Such taxanes may, however, be present in plant materials in relatively small amounts so that, in the case of taxol, for example, large numbers of the slow-growing yew trees forming a source for the compound may be required. The art has thus continued to search for synthetic, including semi-synthetic routes for the preparation of naturally occurring taxanes such as taxol, as well as routes for the preparation of synthetic, pharmaceutically useful analogs thereof.
The present invention provides novel xcex2-lactam compounds of the following formula I: 
where
R1 and R2 are:
(i) both the same alkyl group;
(ii) together form a cycloalkyl group;
(iii) together form a cycloalkenyl group; or
(iv) together form a heterocyclo group;
R3 is alkyl;
R4 is aryl;
R5 is hydrogen, arylcarbonyl, or alkyloxycarbonyl, and salts thereof.
The xcex2-lactams of the present invention are useful as intermediates in the preparation of sidechain-bearing taxanes such as taxol and taxol derivatives. In particular, these compounds may be coupled with a taxane moiety to form the aforementioned sidechain.
As the stereochemistry of taxanes may affect their pharmaceutical activity, it is desirable to employ xcex2-lactam intermediates which will provide the final taxane product with the stereochemistry sought. In the xcex2-lactams of the present invention, the carbon marked with an asterisk in the above formula I is a non-asymmetric carbon. Where such a carbon center is asymmetric, a mixture of diastereomers can be formed. The xcex2-lactams of the present invention provide superior results relative to xcex2-lactams which contain an asymmetric carbon at the corresponding position since, when the latter compounds are prepared, or when they are coupled with a taxane moiety, products are formed as a mixture of stereoconfigurations. The formation of such a mixture of stereoisomers results in an inefficient use of the starting materials, and complicates separation and purification procedures.
The xcex2-lactams of the formula I of the present invention are further advantageous in terms of the yield and purity of the final taxane product. In particular, the xcex2-lactams of the present invention allow efficient conversion, and therefore use of lesser amounts, of starting materials, as well as simplified separation and purification procedures, when employed as intermediates in the preparation of sidechain-bearing taxanes.
The present invention also provides novel methods for using the aforementioned xcex2-lactams of the formula I in the preparation of sidechain-bearing taxanes, and the novel sidechain-bearing taxanes prepared.
The present invention is described further as follows.
The terms xe2x80x9calkylxe2x80x9d or xe2x80x9calkxe2x80x9d, as used herein alone or as part of another group, denote optionally substituted, straight and branched chain saturated hydrocarbon groups, preferably having 1 to 10 carbons in the normal chain. Exemplary unsubstituted such groups include methyl, ethyl, propyl, isopropyl, n-butyl, t-butyl, isobutyl, pentyl, hexyl, isohexyl, heptyl, 4,4-dimethylpentyl, octyl, 2,2,4-trimethylpentyl, nonyl, decyl, undecyl, dodecyl and the like. Exemplary substituents may include one or more of the following groups: halo, alkoxy, alkylthio, alkenyl, alkynyl, aryl, cycloalkyl, cycloalkenyl, hydroxy or protected hydroxy, carboxyl (xe2x80x94COOH), alkyloxycarbonyl, alkylcarbonyloxy, carbamoyl (NH2xe2x80x94COxe2x80x94), amino (xe2x80x94NH2), mono- or dialkylamino, or thiol (xe2x80x94SH).
The terms xe2x80x9clower alkxe2x80x9d or xe2x80x9clower alkylxe2x80x9d, as used herein, denote such optionally substituted groups as described above for alkyl having 1 to 4 carbon atoms in the normal chain.
The terms xe2x80x9calkoxyxe2x80x9d or xe2x80x9calkylthioxe2x80x9d, as used herein, denote an alkyl group as described above bonded through an oxygen linkage (xe2x80x94Oxe2x80x94) or a sulfur linkage (xe2x80x94Sxe2x80x94), respectively. The term xe2x80x9calkyloxycarbonylxe2x80x9d, as used herein, denotes an alkoxy group bonded through a carbonyl group. The term xe2x80x9calkylcarbonyloxyxe2x80x9d, as used herein, denotes an alkyl group bonded through a carbonyl group which is, in turn, bonded through an oxygen linkage. The terms xe2x80x9cmonoalkylaminoxe2x80x9d or xe2x80x9cdialkylaminoxe2x80x9d denote an amino group substituted by one or two alkyl groups as described above, respectively.
The term xe2x80x9calkenylxe2x80x9d, as used herein alone or as part of another group, denotes such optionally substituted groups as described for alkyl, further containing at least one carbon to carbon double bond. Exemplary substituents include one or more alkyl groups as described above, or one or more groups described above as alkyl substituents.
The term xe2x80x9calkynylxe2x80x9d, as used herein alone or as part of another group, denotes such optionally substituted groups as described for alkyl, further containing at least one carbon to carbon triple bond. Exemplary substituents include one or more alkyl groups as described above, or one or more groups described above as alkyl substituents.
The term xe2x80x9ccycloalkylxe2x80x9d, as used herein alone or as part of another group, denotes optionally substituted, saturated cyclic hydrocarbon ring systems, preferably containing 1 to 3 rings and 3 to 7 carbons per ring. Exemplary unsubstituted such groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclodecyl, cyclododecyl, and adamantyl. Exemplary substituents include one or more alkyl groups as described above, or one or more groups described above as alkyl substituents.
The term xe2x80x9ccycloalkenylxe2x80x9d, as used herein alone or as part of another group, denotes such optionally substituted groups as described above for cycloalkyl, further containing at least one carbon to carbon double bond forming a partially unsaturated ring. Exemplary substituents include one or more alkyl groups as described above, or one or more groups described above as alkyl substituents.
The terms xe2x80x9carxe2x80x9d or xe2x80x9carylxe2x80x9d, as used herein alone or as part of another group, denote optionally substituted, homocyclic aromatic groups, preferably containing 1 or 2 rings and 6 to 12 ring carbons. Exemplary unsubstituted such groups include phenyl, biphenyl, and naphthyl. Exemplary substituents include one or more, preferably three or fewer, nitro groups, alkyl groups as described above, or groups described above as alkyl substituents.
The term xe2x80x9carylcarbonylxe2x80x9d, as used herein alone or as part of another group, denotes an aryl group as described above bonded through a carbonyl group.
The terms xe2x80x9cheterocycloxe2x80x9d or xe2x80x9cheterocyclicxe2x80x9d, as used herein alone or as part of another group, denote optionally substituted, fully saturated or unsaturated, aromatic or non-aromatic cyclic groups having at least one heteroatom in at least one ring, preferably monocyclic or bicyclic groups having 5 or 6 atoms in each ring. The heterocyclo group may, for example, have 1 or 2 oxygen atoms, 1 or 2 sulfur atoms, and/or 1 to 4 nitrogen atoms in the ring. Each heterocyclo group may be bonded through any carbon or heteroatom of the ring system. Exemplary heterocyclo groups include the following: thienyl, furyl, pyrrolyl, pyridyl, imidazolyl, pyrrolidinyl, piperidinyl, azepinyl, indolyl, isoindolyl, quinolinyl, isoquinolinyl, benzothiazolyl, benzoxazolyl, benzimidazolyl, benzoxadiazolyl, benzofurazanyl, and especially, tetrahydropyranyl (e.g. 4-tetrahydropyranyl). Exemplary substituents include one or more alkyl groups as described above, or one or more groups described above as alkyl substituents.
The terms xe2x80x9chalogenxe2x80x9d or xe2x80x9chaloxe2x80x9d, as used herein alone or as part of another group, denote chlorine, bromine, fluorine, and iodine.
The term xe2x80x9ctaxane moietyxe2x80x9d, as used herein, denotes moieties containing the core structure: 
which core structure may be substituted and which may contain ethylenic unsaturation in the ring system thereof.
The term xe2x80x9ctaxanexe2x80x9d, as used herein, denotes compounds containing a taxane moiety as described above. The term xe2x80x9csidechain-bearing taxanexe2x80x9d, as used herein, denotes compounds containing a taxane moiety as described above, further containing a sidechain bonded to said moiety at C-13.
The term xe2x80x9chydroxy (or hydroxyl) protecting groupxe2x80x9d, as used herein, denotes any group capable of protecting a free hydroxyl group which, subsequent to the reaction for which it is employed, may be removed without destroying the remainder of the molecule. Such groups, and the synthesis thereof, may be found in xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d by T. W. Greene, John Wiley and Sons, 1981, or Fieser and Fieser. Exemplary hydroxyl protecting groups include methoxymethyl, 1-ethoxyethyl, 1-methoxy-1-methylethyl, benzyloxymethyl, (xcex2-trimethylsilylethoxy)methyl, tetrahydropyranyl, 2,2,2-trichloroethoxycarbonyl, t-butyl(diphenyl)silyl, trialkylsilyl, trichloromethoxycarbonyl, and 2,2,2-trichloroethoxymethyl.
The term xe2x80x9csaltxe2x80x9d, as used herein, includes salts with organic and/or inorganic acids and/or bases.
The term xe2x80x9calkali metal silylamide basexe2x80x9d, as used herein, denotes a base containing the moiety: 
where M is an alkali metal such as lithium, sodium or potassium.
Preferred xcex2-lactams of the present invention are those compounds of the formula I which are crystalline compounds, rather than liquids (oils) at ambient conditions. Such crystalline compounds are advantageous relative to liquid compounds as they may be more easily prepared and obtained in pure form, particularly at larger scales, thus facilitating their subsequent use as intermediates in the formation of sidechain-bearing taxanes such as taxol and taxol derivatives.
Particularly preferred compounds of the formula I are those where R1 and R2 are both the same unsubstituted lower alkyl group, especially where R1 and R2 are both methyl; R3 is unsubstituted lower alkyl, especially methyl; R4 is phenyl; and R5 is hydrogen, benzoyl or t-butoxycarbonyl.
xcex2-lactams of the formula I may be prepared by methods such as those shown in the following Reaction Scheme for the prepartion of cis xcex2-lactams of the formula I. 
The starting compounds of the formula II may be prepared by methods such as those described in U.S. patent application Ser. No. 07/822,015, filed Jan. 15, 1992 by Patel et al., incorporated herein by reference. It is particularly preferred to employ xcex2-lactams which are stereoisomerically (that is, enantiomerically) pure.
The compound of the formula II may be converted to a compound of the formula I by reaction of the former, in the presence of an acid catalyst, with a compound of the formula III or IV: 
where R1, R2 and R3 are as defined above and R1a (i) is a group such that R1axe2x80x94CH2xe2x80x94 is the same as R2 when R2 is alkyl or (ii) forms, together with R2 and the atoms to which R1a and R2 are bonded, a cycloalkenyl group or heterocyclo group containing at least one carbon to carbon double bond. Exemplary compounds of the formula III include the compounds: dimethoxypropane, 
Exemplary compounds of the formula IV include the compounds: 
A particularly preferred method for obtaining a compound of the formula I where R1 and R2 are both the same alkyl is by contacting a compound of the formula II with a compound of the formula IV where R3 is as defined above and R1a is a group such that R1axe2x80x94CH2xe2x80x94 is the same as R2, in the presence of an acid catalyst such as an organic sulfonic acid, for example, pyridinium p-toluene sulfonate (PPTS), toluene sulfonic acid or camphor sulfonic acid. 2-Methoxypropene is preferred as the compound of the formula IV.
The aforementioned reaction is preferably conducted at a temperature of from about xe2x88x9230xc2x0 C. to about 30xc2x0 C., especially at about 0xc2x0 C., and at ambient pressure. The reaction may, for example, be completed over the course of about 0.5 hour to about 10 hours, and is preferably conducted under an atmosphere of inert gas such as argon.
Preferred mole ratios of the compound of the formula III or IV: the compound of the formula II are from about 6:1 to about 1:1. An amount of acid is employed which is effective to catalyze the reaction.
Organic solvents are preferably employed which are inert to the reaction. Particularly preferred solvents are acetone, dimethylformamide, tetrahydrofuran, dichloromethane, acetonitrile and toluene. Amounts of solvents are preferably those where the ratio of compound of the formula II: solvent is from about 1:5 to about 1:40, weight:volume.
The xcex2-lactam of the formula I so obtained, where R5 is hydrogen, may optionally be converted to a xcex2-lactam of the formula I where R5 is arylcarbonyl or alkyloxycarbonyl, with or without prior isolation of the xcex2-lactam where R5 is hydrogen, by contacting the former xcex2-lactam where R5 is hydrogen with a compound of the formula V or VI:
R6xe2x80x94C(O)xe2x80x94Xxe2x80x83xe2x80x83(V)
or
R6xe2x80x94C(O)xe2x80x94Oxe2x80x94C(O)xe2x80x94R6xe2x80x83xe2x80x83(VI)
where
R6 is aryl or alkoxy; and
X is halo, especially chloro.
The above reaction is preferably conducted in the presence of a tertiary amine such as diisopropyl(ethyl)amine, triethylamine and 4-dimethylaminopyridine. Benzoyl chloride is preferred as the compound of the formula V, especially for the preparation of taxol. BOC anhydride (compound VI where R6 is t-butoxy) is preferred as the compound of the formula VI, especially for the preparation of taxotere.
In the above reaction, it is preferred to employ temperatures of from about xe2x88x9230xc2x0 C. to about 30xc2x0 C., especially about 0xc2x0 C., and ambient pressure. The reaction may, for example, be completed over the course of about 2 hours to about 10 hours, and is preferably conducted under an atmosphere of inert gas such as argon.
Preferred mole ratios of the compound of the formula V or VI: xcex2-lactam of the formula I where R5 is hydrogen are from about 1:1 to about 5:1. Preferred mole ratios of tertiary amine: xcex2-lactam of the formula I where R5 is hydrogen are from about 1:1 to about 5:1.
Organic solvents are preferably employed which are inert to the reaction. Particularly preferred solvents are methylene chloride, tetrahydrofuran, acetonitrile, acetone, dimethylformamide and toluene. Amounts of solvents are preferably those where the starting xcex2-lactam is from about 15% to about 80% by weight, based on the combined weight of solvent and starting xcex2-lactam.
xcex2-lactams where R5 is not hydrogen are preferred for use in the coupling methods described following.
Taxanes are diterpene compounds containing the taxane moiety: 
described above. Of particular interest are taxanes containing a taxane moiety in which the 11,12-positions are bonded through an ethylenic linkage, and in which the 13-position contains a sidechain, which taxanes are exemplified by taxol. Pharmacologically active taxanes such as taxol may be used as antitumor agents to treat patients suffering from cancers such as breast, ovarian, colon or lung cancers, melanoma and leukemia.
The present invention provides a novel method for the preparation of sidechain-bearing taxanes by coupling a xcex2-lactam of the present invention to form said sidechain. In particular, the present invention provides a novel method for the preparation of a sidechain-bearing taxane of the following formula VII or a salt thereof: 
where R1, R2, R3, R4 and R5 are as defined above, and T is a taxane moiety bonded directly at C-13 of said moiety;
comprising the step of contacting a xcex2-lactam of the formula I or salt thereof of the present invention with a taxane compound of the following formula VIII or salt thereof:
HOxe2x80x94Txe2x80x83xe2x80x83(VIII)
xe2x80x83where T is as defined above, in the presence of a coupling agent; and, optionally, converting the group xe2x80x94OC(R1) (R2) (OR3) of said compound of the formula VII to hydroxyl, thereby forming a sidechain-bearing taxane or a salt thereof of the following formula IX: 
The addition of a sidechain as described above, in and of itself, may impart an increased or more desirable pharmacological activity to the taxane product, or may form a taxane product which is more readily converted to a taxane having an increased or more desirable pharmacological activity than the starting compound. Exemplary taxanes which may be prepared by the present method for the preparation of a sidechain-bearing taxane include those compounds described in European Patent Publication No. 400,971, U.S. Pat. Nos. 4,876,399, 4,857,653, 4,814,470, 4,924,012, and 4,924,011, all incorporated herein by reference. It is preferred to prepare taxotere having the following structure: 
or, most preferably, taxol as the compound of the formula IX.
Exemplary compounds of the formula VIII, having the OH group bonded directly therein at C-13, which may be employed in the method of the present invention are described in the aforementioned documents incorporated by reference, especially in European Patent Publication No. 400,971. Most preferably, the compound of the formula VIII is a compound of the formula X: 
where
R7 is hydrogen, alkylcarbonyl, or a hydroxyl protecting group, especially acetyl; and
R8 is hydrogen or a hydroxyl protecting group; and particularly is a 7-O-trialkylsilyl baccatin III such as 7-O-triethylsilyl baccatin III or 7-O-trimethylsilyl baccatin III. 7-O-triethylsilyl baccatin III may, for example, be obtained from 10-deacetyl baccatin III as described by Denis et al., J. Am. Chem. Soc., 110, 5917 (1988), incorporated herein by reference. 7-O-Triethylsilyl baccatin III is preferably prepared by the methods of the Examples herein. For example, ultimately, where R7 is hydrogen, compound (X) may be acylated in situ before sidechain coupling.
The coupling agent employed in the method of the present invention may be any agent facilitating coupling to form the sidechain-bearing taxane of the formula VII, exemplified by tertiary amines such as triethyl amine, diisopropyl(ethyl)amine, pyridine, N-methyl imidazole, and 4-dimethylaminopyridine (DMAP), and metallic bases allowing formation of a C-13 metal alkoxide on the taxane of the formula VIII such as lithium diisopropylamide (LDA), unsubstituted lower alkyl lithium compounds, or phenyllithium.
Preferably, the coupling agent of the present method is an alkali metal silylamide base or a sterically hindered alkali metal amide base. Exemplary such bases are those of the formula XI: 
where
R9 and R10 are trialkylsilyl, cycloalkyl, or together with the nitrogen atom to which they are bonded, form a heterocyclo group; and
M is an alkali metal, such as lithium, sodium or potassium.
Preferred bases, particularly alkali metal silylamide bases of the formula XI, are those soluble in the reaction medium employed, and are most preferably an alkali metal hexamethyl disilazide (R9 and R10 are trimethylsilyl and M is sodium, lithium or potassium), especially lithium hexamethyldisilazide (LHMDS). xe2x80x9cSterically hindered alkali metal amide basesxe2x80x9d include those bases containing the group xe2x80x94N(M)xe2x80x94 where M is as defined above and which are substantially the same as, or more, sterically hindered than lithium hexamethyldisilazide in the coupling of a xcex2-lactam to the C-13 hydroxyl group-containing taxane compound. Exemplary sterically hindered such bases include alkali metal tetramethyl piperidides and alkali metal dicyclohexylamides.
The aforementioned alkali metal bases, especially silylamide bases of the present method, are advantageous in that they are not strongly nucleophilic, so that degradation of the taxane starting material of the formula VIII is minimized or eliminated, and in that they provide a high yield (preferably, greater than or equal to about 90%) and purity (preferably greater than or equal to about 98%) of taxane product. The present invention further provides a method wherein a taxane of the formula VIII is coupled with any suitable xcex2-lactam providing a sidechain at C-13 of said taxane, including but not limited to the xcex2-lactams of the present invention, wherein an alkali metal silylamide base or a sterically hindered metal amide base is employed as a coupling agent for said coupling.
The above coupling method of the present invention is preferably conducted at a temperature of from about xe2x88x9270xc2x0 C. to about 25xc2x0 C., especially from about xe2x88x9230xc2x0 C. to about 0xc2x0 C., and at ambient pressure. The reaction may, for example, be completed over the course of about one-half hour to about four hours, and is preferably conducted under an inert atmosphere such as argon.
Preferred mole ratios of taxane starting compound of the formula VIII: xcex2-lactam are those greater than about 1:1.6, most preferably from about 1:1 to about 1:1.3, especially about 1:1.2. Preferred mole ratios of taxane starting compound of the formula VIII: alkali metal base, such as silylamide base, are from about 1:1.1 to about 1:1.5, especially about 1:1.1.
Organic solvents are preferably employed which are inert to the reaction. Particularly preferred solvents are tetrahydrofuran (THF), toluene and ether. Amounts of solvents are preferably those where the ratio of starting taxane of the formula VIII to solvent is from about 1:1 to about 1:5, preferably 1:2.5, weight:volume.
The method of the present invention further comprises, subsequent to the reaction forming a sidechain-bearing taxane of the formula VII, optionally converting the group xe2x80x94OC(R1) (R2) (OR3) to hydroxyl. These groups may optionally be converted to a hydroxyl group sequentially or simultaneously with other hydroxyl protecting groups, such as those on the taxane moiety, by suitable means, such as by contact with an acid, for example, an inorganic acid such as HCl or HF, or organic acids such as acetic acid and the like.
Preferably, deprotection is conducted at a temperature of from about xe2x88x9230xc2x0 C. to about 60xc2x0 C., especially at about 0 to 25xc2x0 C., and at ambient pressure. The reaction may, for example, be completed over the course of about 2 hours to about 72 hours, and is preferably conducted under an inert atmosphere such as argon.
Preferred mole ratios of acid for deprotection: taxane are from about 1:1 to about 20:1 (volume:weight). Organic solvents are preferably employed which are inert to the reaction. Particularly preferred solvents are an ethanol/tetrahydrofuran mixture or acetonitrile, acetone and water. Amounts of solvents are preferably those where the taxane is from about 1:10 to about 1:50, preferably 1:30, ratio of taxane: combined solvent, weight:volume (especially, tetrahydrofuran/ethanol and HCl/water).
The present invention also provides the novel sidechain-bearing taxanes of the formula VII and salts thereof described herein.
Taxol is preferably ultimately prepared as the sidechain-bearing taxane by the methods of the present invention. Taxol may be prepared, for example, by contacting a 7-O-trialkylsilyl baccatin III such as 7-O-triethylsilyl baccatin III, as the formula VIII compound, with (3R-cis)-1-benzoyl-3-(1-methoxy-1-methylethoxy)-4-phenyl-2-azetidinone, as the xcex2-lactam, preferably in the presence of an alkali metal silylamide base. The triethylsilyloxy and 1-methoxy-1-methylethoxy groups may be converted to hydroxyl groups subsequent to sidechain formation, by deprotection methods such as those described above, to form taxol.
Salts or solvates such as hydrates of reactants or products may be employed or prepared as appropriate in any of the methods of the present invention.
As can be appreciated, the xcex2-lactams and taxanes described herein may be present in more than one stereoisomeric form. All stereoisomers of the compounds described herein are contemplated, either alone (i.e., substantially free of other isomers), or in admixture with other selected (e.g. as a racemate) or all other stereoisomers. It is preferred that these compounds be substantially free of other isomers, that is, enantiomerically pure.
Preferred stereoconfigurations of the compounds of the formula I are those where the groups xe2x80x94OC(R1) (R2) (OR3) and R4 are in the cis position, that is, 
particularly where the compound of the formula I has the same absolute stereoconfiguration as the compound (3R-cis)-1-benzoyl-3-(1-methoxy-1-methylethoxy)-4-phenyl-2-azetidinone.
Preferred stereoconfigurations of the C-13 sidechains of the compounds of the formulae VII and IX correspond to the stereoconfiguration of the aforementioned cis xcex2-lactams, that is, 
which sidechains have the same absolute stereoconfiguration as that of taxol.